Electrodes



Nov. 7,v 1967 E. M. Jos'r 3,351,439

ELEcTRoDEs Filed oct. 24, 1965 United States Patent O 3,351,439ELECTRODES Ernest M. `lost, Attleboro, Mass., assigner to TexasInstruments Incorporated, Dallas, Tex., a corporation of Delaware FiledOct. 24, 1965, Ser. No. 504,340 3 Claims. (Cl. 2.9-182.2)

ABSTRACT OF THE DISCLOSURE An electrode of the plaque type forbatteries, fuel cells and the like is made by sintering a highly porousmass of loose nickel flakes to form welds at spots between the flakesthereby forming a highly porous structure. According to one form of theinvention, this is impregnated with a liquid suspension of fine Raneynickel-powder particles. Raney nickel is an alloy of nickel andaluminum. The impregnated structure is then dried and further sinteredto weld the fine Raney nickel particles to the nickel flakes. Theresulting skeleton structure is immersed in hot sodium hydroxide toleach out the aluminum from the Raney nickel alloy particles, thusleaving welded to the flakes small nickel particles of sub-micron size.During the process, either before the impregnating step or after the`leaching step, the structure is compressed. As a result,

some, but not all, of the welded nickel flakes form tabularly shapedpores, thereby to obtain a good pore-size distribution in the plaque.

A second form of the invention is like the first form in that asintered, nickel-flake skeleton is constructed. This is impregnated withnickel hydroxide and then subjected to a hot reducing` atmospherecontaining hydrogen to produce fine nickel particles of sub-micron sizewelded to the plate. The resulting water is evaporated. During theprocess either before impregnation or after conversion of the nickelhydroxide to nickel particles on the flakes, the skeleton is compressed,as in the first form of the invention.

A third form of the invention is like the first and second forms in thata sintered, nickel-flake skeleton is constructed. This skeleton iscompressed and then immersed in a nickel-plating solution withapplication of a high current density so as to plate finely dividedparticles in the form of nickel dendrites on the nickel flakes.

Among the several objects of the invention may be noted the provision ofelectrodes of the plaque type having much larger surface areas forgreater catalytic activity than were heretofore available; the provisionof nickel electrodes of the class described having optimum pore size andimproved pore-size distribution obtainable 'by convenient manufacturingsteps; and the provision of lightweight and compact electrodes of thisclass for use in making light-weight, compact high-capacity batteries,fuel cells and the like. Other objects and yfeatures will be in partapparent and in part pointed -out hereinafter.

The invention accordingly comprises the products and methods hereinafterdescribed, the scope of the invention being indicated in the followingclaims.

In the accompanying drawings, in which several of various possibleembodiments of the invention are illustrated,

FIG. l is a diagrammatic cross section illustrating a step used inconstructing a first embodiment of the invention;

FIG. 2 is a view similar to FIG. 1, illustrating subsequent steps usedin constructing the first embodiment;

FIG. 3 is a greatly enlarged fragmentary sectional detail illustratingcertain nickel flakes with finely divided nickel particles weldedthereto;

3,351,439 Patented Nov. 7, 1967 ICC FIGS. 4 and 5 are views similar toFIGS. 1 and 2, respectively, illustrating a second embodiment of theinvention;

FIGS. 6 and 7 are similar to FIGS. 1 and 2, respectively, illustrating athird embodiment of the invention; and

FIG. 8 is a greatly enlarged fragmentary sectional detail of certainnickel flakes with nickel dendrites electroplated thereon.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

Presently known plaque-type electrodes composed of nickel in particulateform such as carbonyl-nickel powder or nickel flakes have surface areasof a few hundred to a few thousand square centimeters per gram. Suchareas are too small to provide the high catalytic activity desired,which requires several tens of thousands of -square centimeters pergram. The surface areas are insuflicient because of the comparativelylarge sizes of the powder particles and flakes. Thus carbonyl-nickelpowder has grain sizes of greater than a micron and the conventionalflakes are on the order of .00004 inch thick and about .004 square inchin area. The apparent density of nickel flakes is smaller than one tenthof a gram per cubic centimeter.

Fine powders which have high surface areas, such as Raney nickel powderswhich have grains of sub-micron size, have been suggested. Raney nickeldesignates a series of alloys of nickel and aluminum. This powder alsois not suitable when used along because of handling diflculties duringfabrication, inferior pore-size distribution, and a tendency to losecatalytic activity with the heat treatment employed during manufacture.

It has been suggested to use so-called double-skeleton electrodes. Theseare composed, for example, of both the coarser carbonyl-nickel powderand the finer Raney nickel powder. While such electrodes have someadvantages, they are not ideal, again because of handling diillcultiesand the fact that they require a considerable amount of thecomparatively large-grained carbonyl-nickel powder, which acts as anundesirable dead Weight. Moreover, the pores are small than desirable.

According to a first form of my invention, a highly porous mass of looseconventional nickel flakes are sintered in a suitable mold at, say, 1000C. for several hours. The mold may be in the form of a conventional flatrectangular carbon dish, for example. Little or no compac-tion isapplied to the flakes in the mold before they are sintered therein. Theresulting sinter-weldexl mass after removal from the mold isself-supporting. The porosity of this sintered mass is as high asapproximately 99%. Its general shape may be flat, such as suggested bythe cross section shown in FIG. 1. O-ther shapes are not precluded. Theremoved sintered mass is then impregnated, by dipping or the like, witha suitable liquid suspension of fine Raney nickel-powder particles. Thesuspension may for example be a mixture of l500 grams of Raney nickelpowder with 400 ml. of a 2% aqueous solution polyethylene oxide orpolyvinyl alcohols. T-he liquid-impregnated structure is then dried. Theresult is a substantially even distribution of the very fine Raneynickel particles deposited on the surfaces of the sintered orsinter-welded nickel flakes. Thereafter the dried structure is sintered,again for example at 1000 C. for several hours, so that the Raney nickelparticles become welded to the nickel flakes. At this stage the mass ofnickel flakes is in the nature of a skeleton forming a thick,low-density plaque `carrying in its pores the Raney nickel particlessintered to the internal surfaces of the flakes. The result is asillustrated in FIG. 1, in which numeral 1 indicates the low-density,highly porous, impregnated, dried and sintered comparatively thickplaque which is self-supporting and of comparatively large volume. Theshort curved lines represent the porous nickel flakes, which constitutea plaque-forming skeleton formed by the sintered nickel Hakes. The dotsindicate the particles of Raney nickel powder sintered on the flakeswithin the pores.

Next, as shown in FIG. 2, the plaque is compressed to a smaller volumeof desired thickness or shape, the particlecarrying akes being pressedtogether. This improves the pore shape-s and their distribution. Anyconventional pressing means may be employed such as a platen (notshown). The compressed plaque is numbered 3 in FIG. 2. Its thickness maybe about one-fifth of that shown in FIG. l, but this is subject tovariation. At this stage the porosity may still be as high asapproximately 85 In FIG. 2 the short curved lines represent the nickelflakes and the dots indicate the Raney nickel powder welded to theflakes in the pores, except that in this case the plaque is somewhatImore densely compacted and of less volume. However, its porosity isstill considerable'. Thus at the stage of FIG. 2, the particles of thisRaney nickel alloy are substantially evenly distributed in particulateform and in welded attachment to the nickel flakes in theirconfigurations as compressed.

Next the pressed structure shown in FIG. 2 is immersed in hot sodiumhydroxide (NaOH) or potassium hydroxide (KOH) whereby the aluminum isleached out of the Raney nickel alloy, leaving extremely small particlesof nickel (Ni) attached to the nickel fiakes, the conversion from Raneynickel alloy to Ni being accomplished in situ on the nickel flakes. As aresult, each nickel ake 5 (several of which are diagrammatically shownin cross section in FIG. 3 on a greatly enlarged scale) has attached orwelded to its surfaces exceedingly fine' particles of nickel, numbered7. These leached nickel particles are very small, being mostly ofsubmicron size. The very small particles 7 are in permanent attachmentto their supporting nickel flakes and are well distributed in a goodpore distribution. The pores are suggested at numeral 9 in FIG. 3. Theresulting surface area for catalytic activity provided by the very fine,well-distributed, welded nickel particles carried by the plaque-formingwelded fiakes is of the order of several tens to approximatelytwo-hundred square meters per gram weight of finished electrodematerial. The catalytic activity of the finished electrode is extremelyhigh when used, forexample, in the electrolyte of a battery or a fuelcell.

It will be understood that the compression step alternatively may beplaced ahead of the step of dipping into the Raney nickel suspension orafter the leaching step.

Another form of the invention is as follows. A highly porousnickel-flake sintered plaque is prepared in thick form as abovedescribed, with the result as illustrated at 11 in FIG. 4. Again this isthen compressed as above described and as illustrated in FIG. 5. Thecompressed nickel-ake plaque is then impregnated with nickel hydroxideNi(OH)2 as by dipping or the like so that after drainage of excessliquid, the surfaces of the akes are wet with the nickel hydroxide. Thewet plaque is then placed in a hot reducing atmosphere containinghydrogen. By this means the nickel hydroxide is converted to extremelyfine particles of nickel attached or welded to the plate, along with theproduction of water, which evaporates. The reaction formula is It ispossible to use a temperature as high as on the order of 750 C. but thisdepends upon the wetness or dryness of the hydrogen atmosphere. If thepressure of the oxygen contained in the water is, for example, 3 1019mm. of mercury, an appropriate temperature to be used during reductionmay be on the order of 300 C. This corresponds to a dewpoint of 50 C.`Other values may be used, depending upon -the speed of reactiondesired. For example, a lower temperature such as 200 C. may be used ifthe reducing hydrogen atmosphere is dry enough.

After drying, the resulting porous nickel plaque with the fine nickelparticles bonded to the Hakes in the pores is pressed to decrease thebulk volume, as illustrated at 13 in FIG. 5. A 5:1 thickness ratio mayagain be employed. An enlarged section of FIG. 5 will appear much likethe section diagrammed in FIG. 3, the only difference being in how thenickel particles 7 have been produced in situ on the sintered nickelakes. Again they are mostly of submicron size and are metallurgicallybonded in particulate form to the flakes.

It will be understood that compression may precede rather than followthe impregnation and reduction steps, if desired.

The third form of the invention is illustrated in FIGS. 6, 7 and 8. Inthis case a highly porous sintered nickelfiake plaque is prepared asabove described `and as illustrated at 15 in FIG. 6. It is thencompressed as illustrated at 21 in FIG. 7, at, say, a 5:1 ratio. It isthen immersed in a conventional nickel-plating bath at a high currentdenisty, so as to plate finely divided particles in the form of nickeldendrites 17 on the nickel flakes 19 (FIG. 8). A current density of lamp. per sq. cm. is satisfactory. Since appropriate nickel-plating bathsare well known to those skilled in the art, further description in thisregard will be unnecessary. While the dendrites are not of submicronsize in the sense that particles 7 are, nevertheless they have very highsurface areas because of their thin filamentary character. In this formof the invention compression preferably precedes plating so as to avoidcrushing of plated dendrites, but it will be understood that this ordermay be reversed and most of the advantages of the invention retained.

According to all three forms of the invention above described, therewill be produced a compact sinteredY nickel-tiake skeleton havinginternal pore surfaces cov ered with metallurgically bonded very finelydivided nickel particles having extensive surface areas. The porescombine favorable sizes and distribution in the plaque.

It will be seen from FIGS. 3 and 8 that some, though not all of thenickel flakes 5, in view of the leaf-like shapes, tend to becomeoriented somewhat flatwise relative to one another, which producesfavorable elongate or tabular shapes of some of the pores 9. This, alongwith the fact that some of the pores will have other shapes, forms agood pore-size distribution and consequently a good distribution of thenickel particles metallurgically bonded to their inner surfaces.Moreover, because of the metallurgical bonding, these distributionsremain permanently throughout the life of the electrode when in use.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above products and methodswithout departing from the scope of the invention, it is intended thatall matter contained in the `above description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:

1. A porous electrode comprising a mass of nickel flakes, permanentmetallic connections between points of contact of the flakes to form askeleton, the mass of connected flakes being in the form of a plaque,nickel particles distributed substantially over the surfaces of theconnected flakes, metallic connections between said particles and thesurfaces of said flakes, some of the connected flakes being located toform between them a substantial number of tabularly shaped pores, thepore volumes between other connected flakes being of other than tabularshapes, thereby to effect a good pore-size distribution in the plaque.

2. A porous electrode according to claim 1 wherein said particles are inthe form of nickel dendrites.

3. A porous electrode comprising a mass of nickel akes, welds betweenpoints of contact between the flakes 5 6 to forma skeleton, the mass ofwelded akes being in the 2,627,531 2/ 1953 Vogt. form of a plaque,nickel particles substantially on all of 2,700,062 1/ 1955 Schlecht75-222 X which are of sub-micron size, said particles being distrib-3,073,697 1/ 1963 Friese 29-1822 X uted over substantially the entiresurfaces of the welded 3,150,011 9/ 1964 Winsel 75-208 X flakes, Weldsbetween said particles and said surfaces, 5 3,214,355 10/ 1965 Kandler136-120 X some of the Welded flakes being located flat-Wise relative toone another to form substantially tabularly shaped FOREIGN PATENTS poresbetween them, the pores between other welded 540,447 5 /1957 Canadaflakes having other shapes to effect a good pore-size dis- 1,116,28711/1961 Germany. tribution in the Plaque- 10 868,837 5/1961 GreatBritain. References Cife Great Britain. UNITED STATES PATENTS BENJAMINR. PADGETT, Primary Examiner. 2,180,988 ll/ 1939 Lemmers 75-222 X2,389,060 11/1945 Kurtz 75 222 X A. I. STEINER, Asszstant Exammer.

1. A POROUS ELECTRODE COMPRISING A MASS OF NICKEL FLAKES, PERMANENTMETALLIC CONNECTIONS BETWEEN POINTS OF CONTACT OF THE FLAKES TO FORM ASKELETON, THE MASS OF CONNECTED FLAKES BEING IN THE FORM OF A PLAQUE,NICKEL PARTICLES DISTRIBUTED SUBSTANTIALLY OVER THE SURFACES OF THECONNECTED FLAKES, METALLIC CONNECTIONS BETWEEN SAID PARTICLES AND THESURFACES OF SAID FLAKES, SOME OF THE CONNECTED FLAKES BEING LOCATED TOFORM BETWEEN THEM A SUBSTANTIAL NUMBER OF TABULARLY SHAPED PORES, THEPORE VOLUMES BETWEEN OTHER CONNECTED FLAKES BEING OF OTHER THAN TABULARSHAPES, THEREBY TO EFFECT A GOOD PORE-SIZE DISTRIBUTION IN THE PLAQUE.